1. Field of the Invention
This invention relates to a method of manufacturing a vibrating structure gyroscope having a silicon substantially planar ring vibrating structure and capacitive means for imparting drive motion to and sensing motion of the vibrating structure. Such a manufacturing method is particularly suitable for producing a vibrating structure gyroscope by micromachining.
2. Discussion of Prior Art
Micromachined vibrating structure gyroscopes are capable of being produced in high volumes and at low unit cost. This has opened up a diversity of new commercial application areas such as automobile chassis control and navigation systems and camcorder stabilization.
Micromachined vibrating structure gyroscopes are capable of being produced in high volumes and at low unit cost. This has opened up a diversity of new commercial application areas such as automotive chassis control and navigation systems and camcorder stabilisation.
Conventional vibrating structure gyros may be constructed using a variety of resonator designs. These include vibrating beams, tuning forks, cylinders, hemispherical shells and rings. Due to the inherently planar nature of micromachining processes, not all of these structures are suitable for micro-fabrication. The wafer processing techniques give high dimensional tolerancing and alignment accuracies in the plane of the wafer. For ring structures all of the resonance motion is in the plane of the ring and hence it is these dimensions which are most critical to device performance. Planar ring structures are thus particularly suitable for production using these methods and a number of designs are known. These include the inductively driven and sensed devices described in EP-B-0619471, EP-A-0859219. EP-A-0461761 and U.S. Pat. No. 5,547,093 additionally describe devices which are driven and sensed capacitively.
In the previously proposed inductive devices the resonator structures are etched from crystalline Silicon wafers. However, they require the application of a magnetic field to provide the transducer functions. This is facilitated by the use of a magnetic circuit incorporating a permanent magnet and shaped pole pieces. These must be constructed using conventional fabrication techniques and require subsequent assembly and accurate alignment to the resonator structure. This limits the degree of device miniaturization that is possible and adds significantly to the unit cost.
The device described in EP-B-0619471 is also etched from a crystalline Silicon wafer but has the advantage that the drive and pick-off transducer structures are fabricated using wafer processing and assembly techniques and do not require additional, non-micromachined components to operate. The design and fabrication method is thus compatible with a device size significantly smaller than the inductive devices. The design employs a stack of three bonded wafers which must be individually processed and accurately aligned. The transducer gains, and hence the device performance, will be dependent upon the depth of the cavity formed between the wafers. While the micro-fabrication processes provide accurate alignment and tolerancing in the plane of the wafer, control of dimensions in this third axis is less accurate resulting in an inevitable variability in device characteristics. A further disadvantage of this device is the large number of fabrication steps and the requirement for double sided wafer processing. Therefore, while this design does result in a potentially small device which eliminates the requirement for the fabrication and assembly of magnetic circuit components, the complex fabrication will still result in a high unit cost.
The device described in U.S. Pat. No. 5,547,093 also has drive and pick-off transducer structures produced using wafer processing techniques and is capable of fabrication in small size. This design has the additional advantage that the critical transducer gaps are in the plane of the wafer and thus accurately controlled. However, unlike the previous devices the resonator in this instance is constructed from electroformed metal. For the devices etched from crystalline Silicon, the mechanical properties of the material from which the resonator is formed are unaffected by the fabrication processes. The performance of any vibrating structure gyro is critically dependent upon the nature and stability of the mechanical properties of the resonator. Crystalline Silicon is capable of sustaining high Q oscillations with resonance characteristics which are stable over temperature and is thus an ideal resonator material. Electroformed metals are not capable of matching the near perfect elastic behaviour and uniformity of crystalline Silicon. In order to optimize the deposition process uniformity it is necessary to maintain a constant feature size. This requires the ring and support leg widths of the vibrating structure to be identical and severely restricts the resonator dimensional design flexibility. The modal behaviour of the resulting structure will be dominated by the resonator support legs giving potential mounting sensitivity problems and complicating mode balancing procedures. Fabrication of this structure is a complex procedure involving a large number of process steps which will adversely affect both device wafer yield and fabrication costs.
GB Patent Application No. 9817347.9 describes a capacitively driven and sensed ring vibrating structure or resonator which may be fabricated from bulk crystalline Silicon. This structure is shown in plan view in FIG. 1.
There is a need for a method of manufacturing such a gyroscope to an improved degree of accuracy whilst ensuring that the resulting vibrating structure preserves the mechanical properties of the Silicon.
According to one aspect of the present invention there is provided a method of manufacturing a vibrating structure gyroscope having a silicon substantially planar ring vibrating structure, capacitive means for imparting drive motion to and sensing motion of the vibrating structure, and a screen layer surrounding the capacitive means, including the steps of depositing a first layer of photoresist material on to one surface of a plate like glass or silicon substrate, hardening, patterning and developing the first photoresist layer to expose selected areas of the substrate, etching said exposed areas of the substrate to form cavities therein, stripping the remaining first layer photoresist material from the cavitated substrate, attaching a layer of silicon to the cavitated said one surface of the substrate, depositing a layer of aluminium on the surface of the silicon layer most remote from the surface thereof attached to the substrate, depositing a second layer of photoresist material on to the outermost surface of the aluminium layer with respect to the silicon layer, hardening, patterning and developing the second photoresist layer to expose selected areas of the aluminium layer,etching said exposed areas of the aluminium layer to leave on the silicon layer regions of aluminium providing bond pads for grounding the screen layer, bond pads forming connection points for the capacitive drive and sensing means, and bond pads for electrical connection to the silicon substantially planar ring vibrating structure, stripping the remaining second photoresist layer from the aluminium layer, depositing a third layer of photoresist material on to the silicon layer over the remaining deposited aluminium layer regions, hardening, patterning and developing the third layer of photoresist material to expose selected areas of the silicon layer, deep reactive ion etching the exposed selected areas of the silicon layer to form, from the silicon layer, the capacitive drive and sensing means, the surrounding layer, and the substantially planar ring vibrating structure mounted by a hub above the substrate cavities which permit unrestricted oscillation of the ring structure, and electrically to isolate the capacitive drive and sensing means, screen layer and ring vibrating structure from one another.
Preferably the photoresist material is deposited by spinning and hardened by baking.
Conveniently selected areas of the substrate exposed by developing the first photoresist layer are etched by an isotropic wet etch process.
Advantageously the substrate is made of glass to which the silicon layer is attached by anodic bonding.
Alternatively the substrate is made of silicon thermally oxidized to produce a surface oxide layer to which the silicon layer is attached by fusion bonding.
Conveniently the layer of aluminium is attached to the silicon layer by sputtering.
Advantageously the exposed areas of the aluminium layer are etched by a phosphoric acid based process.